Okay - I just did some testing with my D7000. I used a strong IR filter - a B+W 87C filter which is around 810nm. I also tested using my converted D200 which has a Lifepixel Deep IR filter - approximately 810nm.
I used a Nikon 50mm f/1.4 lens.
The scene was a test landscape scene - 50% blue sky and 50% trees in full sun.
As a baseline, my converted D200 produced a correct exposure at 1/160 sec, f/8, and ISO 200.
My D7000 with no filter produced a correct exposure at 1/200 sec, f/8, ISO 200.
Using Manual Priority and the IS filter at an exposure of 1.3 seconds, f/8, and ISO 200 - the scene was nearly black.
Using Manual Priority, a timed bulb exposure of 60 seconds, f/8, and ISO 200, my scene was underexposed.
Using Manual Priority, a timed bulb exposure of 120 seconds, f/8, and ISO 200, my scene was appropriately exposed.
Now the scene could mean some variation, and a weaker filter would permit more visible light leakage, but it appears that the only way you get 2-4 second exposures is with a wide open aperture, increased ISO - maybe even Auto ISO, and probably some pretty severe light leakage from the visible spectrum.
The Deviantart link posted above in this thread shows a good bit of false IR - a little IR and a lot of leakage from the red end of the spectrum. A pure IR image would have much more contrast between the sky and sunlit grasses.